1. Field of Invention
This invention relates to crush resistant flexible tubing for conveying liquids and gases. The tubing is formed primarily of uncured elastomeric material that, when cured, provides a strong but flexible length of tubing. More particularly, the invention relates to tubing that has external helical corrugations formed along its axial length as well as supplemental reinforcement embedded therein. The invention especially concerns a unique method for making the reinforced tubing.
2. Description of Related Art
Flexible tubing with helical corrugations to provide crush resistance is typically made using either the “cording” method disclosed in U.S. Pat. Nos. 2,832,096; 2,879,953; 2,888,719; 2,909,198; 3,155,757 and 3,635,255 or the “axially extensible form” method disclosed in U.S. Pat. Nos. 3,168,604; 3,304,581; 3,669,586; 3,705,780; 3,809,522; 3,975,129; 4,053,275. These patents are further referred to below and incorporated by reference herein.
The present invention concerns a novel variation of the type of corrugated tubing that may be made using either the “cording” method or the “axially extensible form” method, the tubing of the invention not only having helical corrugations but also having a correspondingly shaped helical reinforcing element embedded therein along its length.
Helically corrugated tubing resulting from the “cording” method is made by first sliding an extruded sleeve of uncured rubber axially over a rotatable mandrel with a continuous thread formed on its outer surface. When in place on the mandrel, the sleeve is forced into the helical groove or root of the thread by wrapping a length of cord around the sleeve as the sleeve rotates with the mandrel. This serves to impart a desired corrugated shape to the uncured rubber sleeve.
The resulting assembly is then removed from its rotary support and placed in an oven or autoclave to cure the rubber sleeve and set the helical corrugations. The cord is removed from the corrugated tubing by placing the assembly back on a rotary support and rotating the mandrel in the reverse direction while unwinding the cord from the cured tube. After the removal of the cord, the corrugated tube length may be removed from the mandrel by introducing air under pressure between the outer surface of the mandrel and the inner surface of the corrugated tube.
Helically corrugated tubing resulting from the “axially extensible form” method is made using an external forming device having axially spaced helical convolutions. These devices are employed with a forming mandrel to produce embryonic corrugations in an uncured rubber sleeve. The external forming member is placed over the forming mandrel on which the uncured rubber sleeve has been positioned and the sleeve is radially expanded with air pressure from within so that the sleeve bulges into the spaces between the helical convolutions. The external forming member and sleeve are then collapsed axially so that the sleeve is axially compressed concertina fashion to produce embryonic helical corrugations in the sleeve. After this, the forming member is axially extended together with the sleeve and the sleeve is removed from the forming mandrel. Then the sleeve is placed on a curing mandrel where it is axially foreshortened into corrugated form with the desired spacing between adjacent helical convolutions.
The tubular product resulting from either of these methods is both flexible and resilient, but also has sufficient crush resistance to satisfy most application. Also, the tube has a wall that defines continuous internal and external threads with alternating crests and roots along the tube length. In other words, the wall portion that defines a crest portion of the external thread also defines, on its opposite side, the root portion of the internal thread. Thus the tube has an interior surface that is corrugated or ribbed as opposed to being relatively smooth.
While an interior surface that is “ribbed” is suitable for many applications, there are some circumstances where a smooth interior surface would be much preferred if not in fact essential.
For example, in the case of scuba gear, a breathing tube with a smooth interior surface would provide a better flow rate and be easier to clean.
Likewise in aerospace applications a breathing tube with a smooth interior surface would be much preferred for the same reasons.
Another example is tubing for use in land re-breathing devices to filter and purify ambient air in a hazardous gas environment.
A particular limitation as to corrugated rubber tubing made in accordance with either the “cording” method or the “axially extensible form” method is the degree of crush resistance that can be obtained. While the tubing has sufficient resilience to return to its normal condition when crushed (e.g. when stepped on) there are some applications where greater resistance to crushing or kinking is necessary. One such application is wiring protection hose.
Another limitation as to the type of corrugated rubber referred to above is that in some applications a combination of requirements cannot be satisfied by any single elastomeric composition. For example in the oil industry, tubing for conveying oil or gasoline must have an inner surface that resists deterioration due to the corrosive effect of various chemicals. In addition the external surface of the tubing must resist breakdown due to ultra-violet radiation and corrosion due to salt water. No single compound is available economically that meets both of these requirements.
The method of the present invention, however, provides a novel type of helically corrugated rubber tubing that meets the requirements described above and affords other features and advantages heretofore not obtainable.